Much of the exhaust gas emissions in today's gasoline engines is produced during a cold start before the catalytic converter is ready for operation. To reduce exhaust gas emissions, the catalytic converter must be heated as quickly as possible. One way to do this is to operate the engine at an elevated idle speed to increase the exhaust mass flow rate and thus intensify the transfer of heat to the exhaust system. The engine is thus operated with a lean fuel/air mixture (lambda>1) to reduce the emission of uncombusted hydrocarbons. Retarding the ignition point after the top dead center deliberately lowers engine efficiency, which further increases the exhaust mass flow rate.
At a preset maximum acceptable idle speed for heating the catalytic converter, the maximum possible exhaust mass flow rate is limited by the maximum possible decrease in efficiency. This maximum possible decrease in efficiency is currently determined directly by the latest possible ignition angle. This means that increasing the exhaust mass flow rate at a preset idle speed is possible only by further retarding the ignition points, it being possible to continue delaying the ignition points only as long as the combustion of the fuel/air mixture in the combustion chamber of the internal combustion engine is still ensured.